The use of stem cells in a variety of medical treatments and therapies is receiving growing attention. Cell expansion systems can be used to grow stem cells, as well as other types of cells, such as bone marrow cells which may include stem cells. Stem cells which are expanded from donor cells can be used to repair or replace damaged or defective tissues and are considered for treating a wide range of diseases. Cell expansion systems (CESs) are used to expand cells and may be used to expand donor stem cells from bone marrow. Stem cells may be grown in hollow fiber bioreactors in a cell expansion system.